There really is no particular tube by itself that's better for classical in my view, it really depends on the circuit the tube is used in, and the characteristics of the speaker the amplifier is driving. If you have an amp that can use different types of tubes, such as my old Jadis JA80s (6550s, EL34s, KT88s, KT90s), even then you can't generalize, as all can shine depending on the classical music you like and the sensitivity of your speakers--for example, for smaller scale pieces without deep bass and with vocalists, I might prefer EL34's, which shine in the midrange in that unit but are weaker in the bass; for Brahms, Mahler and Russian full scale pieces, I'd opt for KT88s or, if they're Tung Sols, 6550s, which are stronger in the bass and have additional power for the dynamic demands of that music. I guess it really would all come down to your auditioning of the amp/speaker combination you are interested in to see if it works. However, for large scale orchestral works, you will need plenty of power and bass response; either look at higher-powered amps or very efficient speakers.
